The invention relates to the field of internal combustion engines
By nature, combustion engines are noise-generating systems. The noise created by an engine can come from various sources, mainly excited by moving parts (crank-train, valve train, gears) and combustion (cylinder pressure, injection). Most of the noise created in an engine (except exhaust and ancillaries noise) originates or results in medium to high frequency vibrations in the engine's structure. Due to the fact that the engine structure is by nature very rigid in order to withstand the considerable forces developed by the engine, those vibrations propagate very easily into the whole structure. Moreover those engine excitations are strongly correlated, generating even more noise. Therefore, it is well known that there is an interest in providing means to lower internal vibrations or to counter the propagation of those vibrations inside engine structure.
One main localization of vibration transfers are the main bearings (crankshaft bearings), where combustion excitations (transmitted by piston and connecting rods, also by skirts and cylinder block) and inertial excitations of crank-train cross each other. Indeed, the internal combustion engine usually comprises a main engine block made of cast metal. As they have large external surfaces and less stiffness than upper part of the cylinder block, skirts are important noise sources. This main block comprises at least one cylinder, but more often four, six or eight cylinders wherein reciprocating pistons are able to travel back and forth along the cylinder axis, thereby providing within that the engine block variable volume combustion chambers in which the combustion process takes place. Each piston is connected to a crankshaft crankpin by a connecting rod which is articulated at its both ends on the piston and on the crankshaft. The crankshaft is mounted on the engine block by a number of main bearing journals so as to be able to rotate around a longitudinal crankshaft axis. The main bearing journals of the crankshaft are located axially between at least two crankpins so as not to interfere with the movements of the crankpins and of the corresponding end of the connecting rod. In modern high-performance engines, such as modern diesel engines, there can be one main bearing journal between each crankpin of the crankshaft. In other words, there can be the same number of main bearing journals as of the number of cylinders, plus one.
According to a usual construction technique, each main bearing journal of the crankshaft is mounted within a main bearing housing via a bearing bush. A main bearing housing is formed for one part directly on the engine block, and for the other part on a bearing cap which is removably attached to the engine block. Each part is usually in the form of a half cylinder oriented along the crankshaft axis. The bearing cap is usually essentially U-shaped, each free end of the U being bolted to the engine block. By construction, the bearing housing and more specifically the bearing cap are located at the lowermost portions of the engine block. Also by construction, the bearing housings have to withstand the complete force generated in the combustion chambers. This force being by nature cyclical, and the bearing housings being spaced from one another, the bearing housings and the bearing caps more specifically, are prone to vibrate,. As discussed above, these vibrations generate noise, but can also be a problem in terms of the proper functioning of the bearing.
In order to reduce vibrations generated at the bearings, various solutions have already been suggested. A first solution widely used is to connect all the bearing caps together by a rigid frame structure (so-called bedplate structure), most often made of metal, this frame structure being in turn tightly connected to the engine block. Thereby, the rigidity of the bearings is substantially increased so that the amplitude of the low frequency vibrations can be decreased. Nevertheless, this solution has the major drawback that the frame structure is rigid, so frequency of main vibrating modes increases and can generate more noise, and moreover tends to propagate bearing vibrations to the whole engine block.
Document FR-2.711.186 discloses an engine wherein the engine block has two sidewalls which extend vertically downwards from the engine block on each side of the crankcase and of the bearing caps. The sidewalls preferably have a dampening structure, and they are designed to be relatively flexible, so as to form a preferred vibration path. The bearing caps are all connected one to another by two rigid bars, forming an intended rigid structure. The lower edges of the sidewalk are connected to the bearing caps by viscous dampeners. Due to the geometry, it is clear that those dampers are mainly subject to traction and compression stresses along a transverse direction.
Document GB-2.105.784 discloses another type of dampening system for the bearing housings. In this document, the upper part of the bearing housing, and not the bearing cap, has a transversely extending protrusion. The dampening system comprises a tubular elastomeric element having an inner tubular ring and an outer tubular ring adhered thereto, the three elements having the same transversal axis. The outer ring is received within a corresponding cylindrical housing formed in the lateral side wall of the engine block which extends on one side of the bearings, said outer ring being in abutment in said housing in the direction of the bearing. A shaft portion extends transversely across the dampening system and abuts against the inner ring on its external side so that, once said the shaft is bolted onto the bearing housing protrusion, said shaft is not only tightly pressed against the protrusion, it also forces the outer ring of the dampening system against its abutment. Due to this construction, the elastomeric tubular ring is subject to shear stresses whenever there is a relative movement of the bearing housing with respect to the side wall along a transverse direction. It has been shown that an elastomeric dampener is efficient over a larger span of frequencies when it is subject to shear stresses rather than subject to traction and compression stresses. Therefore, the dampening system disclosed in the above-mentioned document may be efficient in dampening transverse movements, but it will not be as efficient in all the other directions, especially for vibrations occurring along the longitudinal axis of the crankshaft. Moreover, the dampening system of GB-2.105.784 is quite complex, especially from a manufacturing point of view. Indeed, the vertical side walls have to be provided with the corresponding housings and the geometry of the various components of the dampening system allow only for minimum tolerances in dimensions. Indeed, any variation in the dimension of a component along the transverse direction may result either in the elastomeric ring to be excessively constrained in its working direction, or in the elastomeric ring to be loose. In the first case, excessive wear will occur, while in the second case the elastomeric ring will be of no use at all and will even generate additional noise. Therefore, such a dampening system is very costly to implement (new cylinder block design, assembly time, etc. . . . ).
In view of the shortcomings of the above-mentioned solutions, it is desirable to provide a novel solution to dampen crankshaft bearing vibrations at a very reasonable cost, without having to redesign extensively the engine block and other components involved.
The invention provides, according to an aspect thereof, for an internal combustion engine having an engine block comprising at least one cylinder extending along a cylinder axis and a crankshaft which is mounted on the engine block by at least a first and a second main bearings so as to be rotatable around a longitudinal crankshaft axis, wherein said main bearings comprise each a first bearing portion and a second bearing portion, said second bearing portion being part of a bearing cap, and wherein said bearing cap is fixed on said engine block by fixing means, characterized in that at least the first bearing cap is connected to the engine block or to the second bearing cap by at least one dampening structure, said structure comprising a first support portion fixed on said bearing cap, a second support portion fixed on said engine block or on an adjacent bearing cap, and a dampening portion comprising an elastomeric material which connects the two support portions, and in that the dampening structure is configured so that any relative movement between the bearing cap and the engine block, or between two bearing caps, along a substantially horizontal direction, including longitudinal and transversal directions, results in the dampening portion being subject mainly to shear stress.